Resonant inverters advantageously have low switching losses and low switching stresses. However, resonant operation is complex due to the fast dynamics of the high-frequency resonant tank circuit; and, hence, control is difficult. Disadvantageously, when input power or output load conditions vary, output voltage or current control may not be achieved through the use of usual control techniques. For example, one known resonant inverter output load voltage or current control method is to vary the frequency of the rectangular wave signal applied to the resonant circuit by the inverter via closed loop control. Commonly assigned U.S. Pat. No. 4,541,041, issued on Sept. 10, 1985 to J. N. Park and R. L. Steigerwald, which is hereby incorporated by reference, discloses in part such a frequency control technique. Briefly explained, the resonant nature of the circuit allows for control of output voltage or current through variation of the frequency at which the inverter's controllable switch means operate. Such a frequency control method has been found satisfactory under normal output load conditions for particular types of resonant inverters (i.e., heavy or medium load conditions for a series resonant inverter and light load conditions for a parallel resonant inverter). The drawback to frequency control, however, is that it may be inadequate to maintain a desired output voltage or current under extended output load conditions (i.e., light load or no load conditions for a series resonant inverter and heavy load conditions for a parallel resonant inverter).
In particular, frequency control of a series resonant inverter will normally be adequate to maintain a desired output voltage during heavy or medium load conditions (i.e., low resistance) because under these conditions, a series resonant circuit has a high quality factor Q and thus a good dynamic range of voltage or current change as frequency is varied. However, under extended or light output load conditions (i.e., high resistance), the series resonant circuit exhibits a low quality factor Q and thus a small dynamic range of voltage or current change as a function of frequency. As a result, for a series resonant inverter, it may be impossible to maintain a desired output voltage or current under light load and no load conditions solely with conventional frequency control.
Furthermore, a series resonant inverter typically provides a unique value of voltage gain for each unique set of output load conditions (i.e., output voltage and current). Conventional control strategies, such as the method of frequency control hereinabove described, ensure stability under high gain conditions (i.e., relatively high output current and relatively low output voltage) at the expense of system response under low gain conditions (i.e., relatively low output current and relatively high output voltage). Therefore, it is desirable to provide a resonant inverter control which maintains a substantially constant control system loop gain over a wide range of output load conditions.